1. Field of the Invention
The present invention relates to a photolithographic process for extreme resolution of a track width definition for a read head and, more particularly, to a bilayer lift off photolithographic process which obviates fencing after backfilling with one or more read head layers.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a magnetic disk, a slider that has read and write heads, a suspension arm and an actuator arm that swings the suspension arm to place the read and write heads adjacent selected circular tracks on the disk when the disk is rotating. The suspension arm biases the slider into contact with the surface of the disk or parks it on a ramp when the disk is not rotating but, when the disk rotates and the slider faces the rotating disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic field signals to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of field signals.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonmagnetic electrically nonconductive first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The areal density of a read head is a measure of the number of bits per square inch that the read head is capable of sensing on the rotating magnetic disk. Areal density is a product of linear density, which is the number of bits per inch along a circular track of the rotating magnetic disk, and track width density, which is the number of tracks per inch along a radius of the rotating magnetic disk. The linear density is quantified as bits per inch (BPI) and the track width density is quantified as tracks per inch (TPI).
The track width of a read head is typically formed with a bilayer photoresist. After forming sensor material layers a first layer of inorganic or organic material, such as photoresist, which is a non-actinic polymer, is applied by spin coating on the wafer and then subjected to a soft bake to remove casting solvents. Next, a second inorganic or organic material, such as photoresist, which is an actinic photoresist, is spun onto the wafer and soft baked. Assuming that the second photoresist layer is a positive photoresist the second photoresist layer is light imaged with a mask preventing exposure of the light to the second photoresist layer portion that is to be retained. The first and second photoresist layers are then subjected to a dissolver, which is a basic solution. The dissolver first dissolves the light exposed portions of the second photoresist layer down to the first photoresist layer and then proceeds to dissolve the first photoresist layer causing an undercut below the second photoresist layer. The dissolution is terminated when a desired undercut is obtained with the second photoresist layer overhanging the first photoresist layer on each side of the first photoresist layer. Accordingly, the width of the second photoresist layer defines the track width of the read head and the first photoresist layer permits the first and second photoresist layers to be lifted off with any layers deposited thereon, which is described hereinbelow.
After forming the bilayer photoresist on the sensor material layers ion milling is implemented to remove exposed portions of the sensor material layers leaving a sensor material layer portion below the bilayer photoresist that has a width equal to the desired track width of the read head. The space on each side of the bilayer photoresist is then backfilled with read head layers, such as first and second hard bias layers and first and second lead layers, with the first hard bias layer and the first lead layer abutting a first side surface of the sensor and the second hard bias layer and the second lead layer abutting a second side surface of the sensor. The hard bias layers and the lead layers are typically deposited by ion beam deposition since the deposition is more collimated than sputter deposition. Even so, a portion of the deposition enters the undercut on each side of the bilayer photoresist and overlaps first and second end portions of the sensor. Even when the thickness of the first photoresist layer is kept very thin, such as 700 xc3x85, the penetration of the deposited materials will be about 0.1 xcexcm per side. Accordingly, the length of each undercut cannot be smaller than 0.1 xcexcm for each side. If the length of the undercut is less than 0.1 xcexcm per side two major problems will be present. The first problem is that when an undercut is less than 0.1 xcexcm the deposited material will adhere to the recessed edge of the first photoresist layer causing the deposited material to form a spike or fence which extends upwardly. These fences present a problem when the second read gap layer is formed after completion of the sensor and the hard bias and lead layers. If the second read gap layer is made thick enough to prevent a shorting between the fence and the second shield layer this increases the read gap and thereby decreases the linear read bit density of the read head. The second problem with the undercut having a length less than 0.1 xcexcm per side is that process variations can cause one side of the bilayer photoresist to have an undercut while the other side has no undercut. These factors then limit the track width of the read head to values of 0.3 xcexcm or larger. This prior art photolithographic process has not permitted track widths smaller than 0.3 xcexcm.
The same bilayer photoresist scheme as described hereinabove may be employed except the first photoresist layer below the second photoresist layer in the track width region of the sensor is completely dissolved by the dissolver so that a strip of the second photoresist in the track width region with a width equal to the desired track width of the read head bridges between first and second portions of the bilayer photoresist. This is accomplished by subjecting the bilayer photoresist to the dissolver for a sufficient period of time to completely remove the first photoresist portion below the second photoresist strip in the track width region. This then frees the track width to be reduced to a level sufficient only for the self-support of the second photoresist strip, which is on the order of 0.1 xcexcm. Accordingly, read heads can be made with track widths down to 0.1 xcexcm.
After forming the bilayer photoresist on the sensor material layers with the second photoresist strip bridging across where the sensor is to be formed, ion milling is again implemented to remove exposed sensor material portions on each side of the strip. The milled away areas on each side of the strip are then backfilled with the first and second hard bias layers and the first and second lead layers. Unfortunately, the deposited material will overlap portions of the sensor below the strip which alters the desired track width of the sensor.
In order to overcome the aforementioned problem of a lead layer overlapping the sensor the present invention forms a protective sacrificial layer, such as carbon, on the sensor material layers before forming the bilayer photoresist. After forming the bilayer photoresist and after forming the first and second hard bias layers and the first and second lead layers, a second sacrificial protective layer is deposited on top of the first and second lead layers. After removal of the bilayer photoresist the portions of the first and second hard bias layers and the first and second lead layers that overlap the sensor are not protected by either one of the first and second sacrificial protective layers. Ion milling is then implemented to remove the portions overlapping the sensor while the first and second sacrificial protective layers protect the spin valve sensor and the first and second lead layers respectively. Any remaining first and second sacrificial layers may then be removed by reactive ion etching (RIE), such as oxygen RIE. The read head may then be completed by forming the second gap layer and the second shield layer.
An object of the present invention is to provide a bilayer photoresist mask which does not cause fencing of hard bias and lead materials deposited into undercuts of the bilayer photoresist when the track width of a read sensor is 0.3 xcexcm or less and a scheme to permit removal of lead layer material overlapping the read sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.